The present invention relates generally to workplace air filtration systems and, more particularly, to a modular air filtration unit which employs local dilution ventilation. While the present invention was developed particularly in the context of a textile processing environment wherein objectionable respirable dust includes relatively small amounts of lint fragments and very small (e.g. 15 micron) dust particles, the invention is also applicable to dust control applications in general.
Increasing concern for the medical effects of airborne particulate, gaseous, and radioactive substances has caused industry to improve the quality of workplace air. An important general approach directed to this end is to employ what are commonly termed engineering controls. Examples of engineering controls are: (1) design and operation of production machinery so that unwanted emissions are minimized; (2) capturing the dust near the source(s) of emissions; and (3) utilizing general capture and dilution ventilation. Examples of other approaches are establishing work practices which minimize emissions or respiratory exposure, and requiring personal respiratory protection.
A commonly employed engineering control technique is the use of air filtration devices for the collection and removal of toxic substances. This is especially true for dust or other particulate matter. In general, filtration is required either to meet OSHA standards or industrial hygiene guidelines in the workplace or EPA standards for emissions into the atmosphere. Moreover, increasing energy costs make it increasingly necessary to recirculate workplace air. The alternative of continually bringing outside air into the workplace can be very costly where heating or cooling must be employed. The need for recirculation systems places increased demands on the filtration equipment since such equipment must be both more efficient and more reliable.
Filtration methods for collection and removal of particulate matter are well known in the art and generally fall into two categories: (1) central filtration and (2) modular filtration.
In a central filtration system, dust-laden air is captured or picked up by drawing it into suction duct work. The air is transported to a central filtration unit, such as a bag house or a drum filter, is cleaned, and is then recirculated back to the workplace, discharged into the atmosphere or, usually, a mixture of the two. Thus, to achieve air recirculation in a central filtration system, there must in general exist both collection duct work and return duct work. In some cases, the return duct work may be part of an air-conditioning system. In any case, the air flow rates are very high and energy losses within the duct work are a major fraction of the total energy required.
In a modular filtration system, to which the present invention is primarily directed, a major difference compared to central filtration systems is the elimination of the duct work such that workplace or machinery emissions are captured, filtered, and returned to a local area. This may be termed "local dilution ventilation". Thus, in one particularly useful embodiment of the broad concept of modular filtration, modular filtration units operate simply by generally capturing dusty room air and preferentially redistributing the clean, filtered air over the workers and/or machinery.
Modular filtration systems are known in the art and typically comprise a blower pulling dusty air into one or more stages of filter media and discharging the air back to the workplace. The media choice is influenced by the size, concentration, and type of dust. The media must be cleaned or discarded after loading up and thus leads to a high operating cost which increases with increasing dust capture rate.
Other dust collection systems are known, such as cyclones, scrubbers, and electrostatic precipitators. However, cyclones are ineffective and improper technology for respirable dust. Scrubbers are inapplicable to efficient modular filtration. Electrostatic precipitators are applicable to a restricted class of dusts, and in some explosion or fire-prone environments may not be employed at all.
As will be apparent from the description hereinafter, by the present invention relatively lower-cost, effective and efficient modular filtration units are provided. Considering cost for example, in 1983 the typical installed cost for high-quality central filtration, having discharge air quality similar to that provided by the present invention, is in the order of $4.00 per cubic foot per minute (CFM). For purposes of comparison, the installed cost for systems employing modular air filtration units of the present invention is in the order of $1.36 per CFM.
Moreover, operating costs for both types of system are dominated by the cost of electrical power to operate the blower motors, and the annual power cost per CFM of filtered air when employing modular air filtration units in accordance with the present invention is typically one-fourth that for central filtration. This is due not only to the fact that modular filtration inherently eliminates expensive losses associated with collection and return duct work, but also as a result of more efficient blower operation in accordance with control system aspects of the present invention.
More particularly, for purposes of illustration but not limitation, it is pertinent to consider two typical application areas in the textile industry.
First, it has heretofore been stated by many practitioners of engineering controls for cotton dust that there are no proven methods for controlling workplace respirable dust levels in certain processes, such as spinning, winding, or warping. The reason behind this statement is the extreme difficulty, indeed practical impossibility, of implementing source capture for such machines. For example, a spinning machine has perhaps 100 spindles turning at 12,000 RPM and liberating dust and fiber and respirable fiber fragments in an obviously general manner that defies source capture. In a warper, which comprises a large frame or creel holding several hundred rolls of yarn which are then pulled onto a long beam for subsequent weaving, the emissions problem is similarly general and inadmissable to source capture. General capture using the modular filtration units described herein has proven to be extremely effective, especially when an induced air flow pattern resulting from a high-velocity clean air discharge of the modular air filtration unit is used.
A second application area for modular filtration is to marginal areas which may already have engineering controls. A good example is carding, where dust is effectively removed from the aggressive action on individual fibers. These emissions must therefore be effectively contained either within the equipment or by source capture devices; otherwise, the workplace dust levels would be extremely high. Currently, almost all carding rooms have some type of dust capture system moving typically 500 to 2000 CFM per carding machine. In many applications having 500-1000 CFM, the workplace dust levels resulting from this dust control equipment were well under the former OSHA Standard of 1000 .mu.g/m.sup.3 and many of them had respirable dust levels in the range of 300 to 500 .mu.g/m.sup.3. (It may be noted that the workplace dust levels depend heavily upon the type of stock and the speed at which it is processed, as well as upon the performance of the dust capture system.)
The new Cotton Dust Standard [Dept. of Labor, OSHA. Occupational exposure to cotton dust. Federal Register, pp. 27350-27436, June 23, 1978] calls for a permissable exposure limit (PEL) in this process of 200 .mu.g/m.sup.3. The employer has already invested in dust capture and air filtration equipment and in air-conditioning equipment whose size is related to the filtration equipment. One option for the employer is to rip out a well-designed and properly-operating dust capture and air-conditioning system and install a much more expensive one operating at a much higher air flow. In many cases, this also necessitates upgrading or rebuilding the air-conditioning system. In accordance with the present invention, marginal processes such as summarized above can be brought into control simply by the addition of dilution ventilation via modular filtration.
Moreover, as illustrated next below, in some cases modular filtration designs can follow different design paths to accomplish respirable dust level reduction as the main objective. Alternatively stated, central filtration technology may be inapplicable or ineffective for respirable dust control as opposed to dust associated with waste-handling, the waste involved deriving from the fiber being processed.
In particular, respirable dust concentrations in the workplace generally obey, at equilibrium, ##EQU1## where, M.sub.r is the respirable dust emission rate in gm/min, and Q is the circulating, filtered air flow in m.sup.3 /min. .chi..sub.eq can be reduced only by reducing the respirable emissions M.sub.r or by increasing the ventilating or diluting air flow, Q.
In some processes, for example carding, source capture can effectively reduce the M.sub.r component from machinery emissions. In other processes, especially from spinning through warping, source capture is not practical, as has been recognized in the Cotton Dust Standard and as briefly mentioned above. In these processes the only engineering control measure is increased dilution ventilation, which generally captures the airborne dust and recirculates filtered air to the workplace.
It is most important to appreciate the magnitude of M.sub.r by a simple but realistic example. In a process area with Q=35,310 CFM=1,000 m.sup.3 /min and .chi..sub.eq =350 .mu.g/m.sup.3, ##EQU2## In other words, a quantity of respirable dust small enough to be contained within a salt shaker (21 grams) of respirable dust emitted each hour into a textile workplace having 35,310 CFM of circulating air is responsible for the 350 .mu.g/m.sup.3 respirable dust concentration. If half this small amount of respirable dust could be captured, then the workplace dust concentrations would drop by half, from 350 to 175 .mu.g/m.sup.3, and the workplace would be placed in compliance with the 200 .mu.g/m.sup.3 OSHA PEL.
For contrast, consider a dust collection and filtration system serving carding machinery. If the 35,310 CFM is supplied by an air washer to the workplace and all this air is returned to the air washer by being drawn into a dust collection system serving 35 cards processing 60 pounds per hour and removing approximately 2% of dust, trash, and other wastes, then the 35,310 CFM must transport away a waste component mass ##EQU3## or 19,068/21=908 times as much waste mass per hour as is responsible for the 350 .mu.g/m.sup.3 workplace respirable dust concentration.
It is an underlying recognition of the present invention that the technologies for respirable dust control and for process dust control should be vastly different because (1) the mass emission rates and (2) the particle sizes are vastly different, by orders of magnitude. It is a misapplication of technology to expect that central filtration, which can well handle large quantities of large particles, can effectively and generally apply to the control of micron-sized respirable dust, in grams per hour quantities.